-LRB- CNN -RRB- -- When the average person contemplates the issues surrounding landfills , it 's doubtful they give much consideration to the tons of food that fill them .

Food biodegrades so where is the problem ?

The problem , environmentalists say , is just that . When food rots , it releases methane , a greenhouse gas which the U.S. Environmental Protection Agency -LRB- EPA -RRB- says is 20 times more damaging to the environment than carbon dioxide -LRB- CO2 -RRB- .

Rotting food in a landfill in Canterbury , England .

The developed world chucks out a lot of food . Such is the volume that according to the U.S. Department of Agriculture -LRB- USDA -RRB- , if just 5 percent of Americans ' food scraps were recovered it would represent one day 's worth of food for 4 million people .

The U.N. World Food Programme offers another way of looking at it : It says the total surplus of the U.S. alone could satisfy `` every empty stomach '' in Africa -LRB- France 's leftovers could feed the Democratic Republic of Congo ; and Italy 's could feed Ethiopia 's undernourished -RRB- .

Proportionately , the UK and Japan have traditionally been among the worst offenders worldwide in recent years when it comes to food waste , discarding between 30 and 40 percent of their food produce annually . The figures for how much the U.S. throws out , however , vary considerably depending on whom you ask .

According to the USDA , just over a quarter of the country 's food -- about 25.9 million tons -- gets thrown in the garbage can every year .

But according to a study conducted by the University of Arizona , that figure could be as high as 50 percent , as the University claims that the country 's supermarkets , restaurants and convenience stores alone throw out 27 million tons between them every year -LRB- representing $ 30 billion of wasted food -RRB- .

Either way , it still costs the U.S. around $ 1 billion every year just to dispose of all its food waste , according to the EPA .

But moral and economic issues aside , it is the environmental concerns around food waste that is driving the push for reform on how to treat the problem of leftovers . Methane , the gas food waste produces , traps 23 times as much heat in the atmosphere as the same amount of CO2 , the EPA says . And landfills are the place you will find most of it -- they account for 34 percent of all methane emissions in the U.S. .

The University of Arizona believes that if Americans cut their food waste in half , it would reduce the country 's environmental impact by 25 percent . The UK 's Waste & Resources Action Program -LRB- WRAP -RRB- -- which says the entire food supply chain in the UK contributes 20 percent of its greenhouse gas emissions -- believes that if we stopped throwing out edible food , the impact it would have on CO2 emissions would be the equivalent of taking 1 in 5 cars off the road .

But ironically , one of the solutions to dealing with food waste actually results in a product that could keep cars on the road : Biogas .

Biogas is a by-product of a process called anaerobic digestion -LRB- AD -RRB- . AD is a process where organic matter -- such as food waste -- breaks down in an environment with little or no oxygen , generating a natural gas made up of 60 percent methane and 40 percent CO2 . It is the exact process , in fact , which goes on in landfills . But there is a difference .

Whereas methane can be harmful to the environment in an open setting , such as a landfill , in controlled and closed settings such as a combined heat and power plant , it can be harnessed and converted into biogas , a renewable energy . And that energy can be used to provide heat , light and fuel .

According to a study by the National Society for Clean Air , biogas-fueled cars can reduce CO2 emissions by anything from 75 percent to 200 percent compared to cars powered by fossil fuels .

Most organic matter can be processed with AD . In the UK it is already being used to treat sewage , which Friends of the Earth -LRB- FOE -RRB- says , reduces CO2 emissions by 16 percent compared to traditional sewage treatments . According to the Chartered Institute for Environmental Health , gas from sewage waste and landfills is already being used to provide 650 MW of electricity to the UK 's national grid , representing between 60 and 75 percent of the country 's green energy -LRB- the UK is Europe 's biggest producer of biogas -RRB- .

However , while the potential for food waste-as-energy seems big , the practical applications for it are currently very small -LRB- only 0.4 percent of the UK 's food waste is processed by AD , for example -RRB- , with critics of AD pointing out that the amount food waste can contribute to the energy supply are negligible to say the least .

FOE itself admits that just 0.36 percent of the UK 's electricity needs could be met by AD . And , if 5.5 million tons of food waste was treated by AD -LRB- the majority of the UK 's annual 6.7 million tons of food waste -RRB- it could only generate enough electricity to power 164,000 houses .

That being said , environmentalists will say , that 's much better than getting that electricity from fossil fuels . And there has been a big push , in Europe in particular , to cut back on the amount of biodegradable waste that is being sent to landfills . According to the European Landfill Directive , the amount of biodegradable waste sent to landfills in member countries by 2020 must reach 35 percent of the levels reached in 1995 .

The country that is leading the way in putting its biodegradable waste mountains to good use -- particularly in the world of biogas-powered cars -- is Sweden .

That country -- which plans to eliminate petrol and diesel vehicles from their streets by 2020 -- already has 7,000 biogas cars on the road . It also has 779 biogas buses and the world 's first biogas train , which , according to The Ecologist , cost just 1 million euros -LRB- $ 1.4 million -RRB- to develop . E-mail to a friend

-LRB- Sources : Chartered Institute of Environmental Health ; U.S. Environmental Protection Agency ; Planet Ark ; U.S. Department of Agriculture ; University of Arizona ; World Food Program ; Waste & Resources Action Program -LRB- WRAP -RRB- ; Friends of the Earth -LRB- UK -RRB- ; National Society for Clean Air ; The Ecologist ; Just-food . com ; Food Production Daily ; Endhunger.org -RRB-

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5 percent of American 's leftovers could feed 4 million people for 1 day

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Disposing of food waste costs the U.S. $ 1 billion a year

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Rotting food releases methane , a more potent greenhouse gas than CO2

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Methane can be harnessed to create clean energy for heat , light and fuel